The invention relates to an opto-mechanical scanner having a scanning function and an associated ranging function. The scanning field is obtained by line-scanning and frame scanning means. The ranging field is fixed during the propagation time of a ranging echo. The ranging source is a laser. The beams issuing from the scanning and ranging fields converge on a scanning detector array and on a ranging detector, respectively, which are arranged in the same cooled space.
Thermal imaging systems are known in which the scene is scanned point by point by means of an opto-mechanical scanner. Generally, these thermal cameras are equipped with infrared optical systems of large diameter.
When the thermal camera is used in conjunction with a laser ranging device the optical system of the ranging device also has a large diameter.
In order to reduce the dimensions and price of the arrangement it is advantageous to use the same optical system for scanning and for laser reception. Laser light is emitted externally of the thermal camera. The laser emitter uses a separate optical system whose axis extends parallel to the axis of the optical system for laser reception.
It is also advantageous to arrange the scanning and ranging detectors in the same cooled space, so that only one space has to be cooled.
It is conceivable that the scanning channel and the ranging channel are the same channel from the entrance optical system to the detector. This is possible in the case of scanners having only one scanning direction. In "parallel" scanners, which have as many detectors as there are scanning lines, this is the case because these scanners are of the slow-scanning type.
In "series" or "series-parallel" scanners, which have two scanning directions as in conventional television, line scanning is very rapid and frame scanning is slow. The orientation of the field being scanned varies very rapidly in the line direction, to such an extent that during the propagation time of the laser pulse between its emission and reception the orientation of the laser reception field has changed through an angle equal to several times the laser field. In order to overcome this problem different methods may be adopted.
It is possible, for example, to bypass the line-scanning by means of dichroic plates which reflect the laser radiation and which transmit the remainder of the infrared spectrum.
Alternatively, it is possible to bypass the line scanning sequentially by means of switchable optical elements.
One method has the disadvantage that elements such as the dichroic plates exhibit absorption. The other method has the disadvantage that switchable elements always give rise to synchronization problems.